Talking with some of my friends who are on numerous Forums, they all say a similar thing.
Members join, maybe make one or two posts, then completely disappear, never to be heard from again.
Many times they have found that a particular Forum has a small core of members, e.g. maybe six to twelve people altogether, who post regularly.
This could be a reason for disinterest in those particular cases, where there isn't enough diversity of members to keep their attention.
Another reason could be that they didn't fully check out the Forum's postings before joining, and it turned out to be not what they thought it was going to be like. That was the case with me on that last woman's Forum (wow! I'm so glad I got out of there pretty quick )
It's a difficult one to answer with any degree of certainty.